CN113168112A - Method of measuring focus parameters associated with structures formed using a lithographic process - Google Patents

Abstract

A method of measuring focus parameters associated with forming a structure using a lithographic process, and associated metrology apparatus, are disclosed. The method comprises the following steps: obtaining measurement data relating to cross-polarization measurements of the structure; and determining a value of the focus parameter based on the measurement data.

Description

Method of measuring focus parameters associated with structures formed using a lithographic process

Cross Reference to Related Applications

This application claims priority from european application 18208291.7 filed on 26/11/2018, the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates to a method and apparatus for measuring a pattern applied to a substrate in a lithographic process.

Background

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). In that case, the patterning device (which is alternatively referred to as a mask or a reticle) may be used to generate a circuit pattern to be formed on an individual layer of the IC. Such a pattern can be transferred onto a target portion (e.g., comprising a portion of, a die, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are successively patterned. The known lithographic apparatus comprises: so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time; and so-called scanners, in which each target portion is irradiated by synchronously scanning the substrate parallel or anti-parallel to a given direction (the "scanning" -direction) while scanning the pattern through the beam. The pattern may also be transferred from the patterning device to the substrate by imprinting the pattern onto the substrate.

To monitor the lithographic process, parameters of the patterned substrate are measured. For example, the parameters may include overlay error between successive layers formed in or on the patterned substrate, and critical line width (CD) of the developed photoresist. Such measurements may be performed on the product substrate and/or on dedicated metrology targets. Various techniques exist for measuring microstructures formed during photolithography, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of dedicated inspection tool is a scatterometer, in which a beam of radiation is directed onto a target on the surface of a substrate and properties of the scattered or reflected beam are measured. Two main types of scatterometers are known. A spectral scatterometer directs a broadband radiation beam onto a substrate and measures the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. An angle-resolved scatterometer uses a monochromatic radiation beam and measures the intensity of the scattered radiation as a function of angle.

Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921a1 and US2010201963a 1. The target used by such scatterometers is a relatively large grating, e.g., 40 μm by 40 μm, and the measurement beam produces a spot that is smaller than the grating (i.e., the grating is under-filled). In addition to the measurement of the characteristic shape by reconstruction, such a device can also be used to measure diffraction-based overlay, as described in published patent application US2006066855a 1. Overlay metrology of smaller targets can be achieved using diffraction-based overlay metrology of dark field imaging of diffraction orders. Examples of dark-field imaging measurements can be found in international patent applications WO 2009/078708 and WO 2009/106279, which are hereby incorporated by reference in their entirety. Further developments of the technology are described in the published patent publications US20110027704A, US20110043791A, US2011102753a1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422a 1. These targets may be smaller than the illumination spot and may be surrounded by product structures on the wafer. Multiple gratings may be measured in one image using a composite grating target. The contents of all of these applications are also incorporated herein by reference.

In performing a lithographic process, such as applying a pattern onto a substrate or measuring such a pattern, the process is monitored and controlled using process control methods. Such process control techniques are typically performed to obtain corrections to the control of the lithographic process.

An important parameter to monitor is the focal length of the projection optics on the substrate when the exposure is performed. This focal length may drift over time and across the substrate for a number of reasons (e.g., because the substrate is not perfectly flat). Focus monitoring typically includes measurement structures with assist features or sub-resolution features (smaller than the imaging resolution of the projection optics). These sub-resolution features, although not imaged, affect the host structure by imposing a focal length dependent asymmetry. Thus, measuring this asymmetry (e.g., using a scatterometer) means that the focal length can be inferred. However, this method is difficult to implement for EUV lithography due to the thin resist used. Furthermore, imaging of sub-resolution features is undesirable for a number of reasons. Astigmatism-based focusing techniques have also been developed but cannot be used for product monitoring because it requires astigmatism (aberration optics) in the projection lens during exposure.

It is desirable to address at least some of the problems set forth above.

Disclosure of Invention

In a first aspect of the invention, there is provided a method of measuring a focus parameter associated with forming a structure using a lithographic process, wherein the method comprises: obtaining measurement data relating to cross-polarization measurements of the structure; and determining a value of the focus parameter based on the measurement data.

In a second aspect of the present invention there is provided a computer program comprising program instructions operable, when run on a suitable apparatus, to perform the method of the first aspect.

In a third aspect of the invention, there is provided a processing system comprising a processor and a computer program product comprising the computer program of the second aspect.

In a fourth aspect of the present invention, there is provided a metrology system comprising: a substrate holder for a substrate; an illumination source for illuminating structures on the substrate with radiation having an illumination polarization state selectable between a first polarization state and a second polarization state; a sensor for sensing scattered illumination from the structure with a sensing illumination state selectable between the first and second polarization states; and the processing system of the third aspect.

Further aspects, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described in the present invention. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings contained herein.

Drawings

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 depicts a lithographic apparatus and other apparatus forming a production facility for semiconductor devices;

FIG. 2 includes a schematic diagram of a scatterometer for measuring a target according to an embodiment of the present invention;

FIG. 3 depicts a schematic of global lithography representing the cooperation between three key technologies to optimize semiconductor fabrication;

FIG. 4 includes (a) plots of HH and VV polarization state measurements based on largely unprocessed intensity data, (b) plots of HV and VH polarization state measurements based on largely unprocessed intensity data, (c) plots of HH and VV polarization state measurements based on processed intensity data, and (d) plots of HV and VH polarization state measurements based on processed intensity data; and

FIG. 5 is a flow chart depicting a method according to an embodiment of the present invention.

Detailed Description

Before describing embodiments of the present invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

FIG. 1 shows a lithographic apparatus LA at 200, which is shown as part of an industrial production facility that implements a high volume lithographic manufacturing process. In the present example, the manufacturing process is adapted for the manufacture of semiconductor products (integrated circuits) on a substrate, such as a semiconductor wafer. Those skilled in the art will appreciate that a wide variety of products may be manufactured by processing different types of substrates in variations of this process. The production of semiconductor products is only used as an example of great commercial significance today.

Within the lithographic apparatus (or simply "lithographic tool" 200), a metrology station MEA is shown at 202 and an exposure station EXP is shown at 204. The control unit LACU is shown at 206. In this example, each substrate visits a measurement station and an exposure station to be patterned. For example, in an optical lithographic apparatus, the projection system is used to transfer a product pattern from patterning device MA onto a substrate using the conditioned radiation and the projection system. This transfer is accomplished by forming a patterned image in the layer of radiation-sensitive resist material.

The term "projection system" used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. The patterning device MA may be a mask or a reticle, which imparts a pattern to a radiation beam transmitted or reflected by the patterning device. Well-known modes of operation include a step mode and a scan mode. As is well known, projection systems can cooperate in a variety of ways with support and positioning systems for the substrate and patterning device to apply a desired pattern to a number of target portions across the entire substrate. A programmable patterning device may be used instead of a reticle with a fixed pattern. For example, the radiation may comprise electromagnetic radiation in the Deep Ultraviolet (DUV) or Extreme Ultraviolet (EUV) bands. The present disclosure is also applicable to other types of lithographic processes, for example, imprint lithography and direct write lithography, for example, using electron beams.

The lithographic apparatus control unit LACU controls all movements and measurements of the various actuators and sensors to receive the substrate W and reticle MA and to perform the patterning operation. The LACU also includes signal processing and data processing capabilities for performing desired calculations related to the operation of the device. In practice, the control unit LACU will be implemented as a system with many sub-units, each handling real-time data acquisition, processing and control of subsystems or components within the device.

The substrate is processed at the measurement station MEA before a pattern is applied to the substrate at the exposure station EXP, so that various preparatory steps can be carried out. The preliminary step may include mapping a surface height of the substrate using a level sensor and measuring a position of an alignment mark on the substrate using an alignment sensor. The alignment marks are nominally arranged in a regular grid pattern. However, the marks deviate from the ideal grid due to inaccuracies in generating the marks and also due to deformations of the substrate that occur throughout its processing. Thus, in addition to measuring the position and orientation of the substrate, the alignment sensor must actually also measure the position of many marks across the substrate area in detail, if the apparatus is to print the product features at the correct locations with very high accuracy. The apparatus may be of a so-called dual stage type having two substrate tables, each having a positioning system controlled by a control unit LACU. When one substrate on one substrate table is exposed at the exposure station EXP, another substrate may be loaded onto another substrate table at the measurement station MEA, so that various preparatory steps may be performed. Therefore, the measurement of the alignment marks is very time consuming and providing two substrate tables enables a considerable increase of the throughput of the apparatus. IF the position sensor IF is not able to measure the position of the substrate table when the substrate table is in the measurement station and when it is in the exposure station, a second position sensor may be provided to enable the position of the substrate table to be tracked at both stations.

Within the production facility, the apparatus 200 forms part of a "lithography unit" or "lithography cluster" which also contains a coating apparatus 208, the coating apparatus 208 being used to apply photoresist and other coatings to a substrate W for patterning by the apparatus 200. At the output side of apparatus 200, a baking apparatus 210 and a developing apparatus 212 are provided for developing the exposed pattern into a solid resist pattern. Between all these facilities, the substrate processing system is responsible for supporting the substrate and transferring the substrate from one piece of equipment to the next. These apparatuses, often collectively referred to as coating and development systems or tracks (tracks), are under the control of a coating and development system control unit or track control unit, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via a lithographic apparatus control unit LACU. Thus, different devices may be operated to maximize throughput and processing efficiency. The supervisory control system SCS receives configuration scheme information R which provides in very detail a definition of the steps to be performed to generate each patterned substrate.

Once the pattern has been applied and developed in the lithography unit, the patterned substrate 220 is transferred to other processing equipment such as illustrated at 222, 224, 226. A wide range of processing steps are performed by various equipment in a typical manufacturing facility. For purposes of illustration, in such an embodiment, apparatus 222 is an etch station and apparatus 224 performs a post-etch annealing step. Additional physical and/or chemical processing steps are applied in additional apparatus 226, and so on. Many types of operations may be required to fabricate real devices, such as deposition of materials, modification of surface material properties (oxidation, doping, ion implantation, etc.), Chemical Mechanical Polishing (CMP), and so forth. Indeed, device 226 may represent a series of different processing steps performed in one or more devices. As another example, an apparatus and process steps for performing self-aligned multiple patterning may be provided to produce a plurality of smaller features based on a precursor pattern placed by a lithographic apparatus.

As is well known, the fabrication of semiconductor devices involves multiple iterations of such processes to build up device structures of appropriate materials and patterns on a substrate layer by layer. Thus, the substrate 230 that reaches the lithography cluster may be a newly prepared substrate, or it may be a substrate that has been previously processed in this cluster or completely in another apparatus. Similarly, depending on the desired processing, the substrate 232 exiting the apparatus 226 may be returned for subsequent patterning operations in the same lithographic cluster, the substrate 232 may be designated for patterning operations in a different cluster, or may be a finished product to be sent for dicing and packaging.

Each layer of the product structure requires a different set of process steps, and the devices 226 used at each layer may be completely different in type. Furthermore, even where the process steps to be applied by the apparatus 226 are nominally the same, there may be several presumably identical machines in a large facility working in parallel to perform steps 226 on different substrates. Small setup differences or imperfections between these machines may mean that they affect different substrates in different ways. Even the relatively common steps for each layer, such as etching (device 222), may be carried out by several etching devices that are nominally identical but operate in parallel to maximize throughput. Furthermore, in practice, different layers require different etching processes (e.g. chemical etching, plasma etching) and special requirements (such as anisotropic etching) depending on the details of the material to be etched.

The preceding and/or subsequent processes may be performed in other lithographic apparatus (as just mentioned), and may even be performed in different types of lithographic apparatus. For example, some layers in a device manufacturing process that are very demanding in terms of parameters such as resolution and overlay may be performed in more advanced lithography tools than other layers that are less demanding. Thus, some layers may be exposed in an immersion lithography tool, while other layers are exposed in a "dry" tool. Some layers may be exposed in a tool operating at a DUV wavelength, while other layers are exposed using EUV wavelength radiation.

In order to properly and consistently expose a substrate exposed by a lithographic apparatus, it is desirable to inspect the exposed substrate to measure properties such as overlay error between subsequent layers, line thickness, Critical Dimension (CD), and the like. Thus, the manufacturing facility in which the lithography unit LC is located also includes a metrology system that receives some or all of the substrates W that have been processed in the lithography unit. The measurement results are directly or indirectly provided to the supervisory control system SCS. If an error is detected, the exposure of subsequent substrates may be adjusted, especially if metrology can be done quickly and quickly enough so that other substrates of the same batch remain to be exposed. In addition, exposed substrates may be stripped and reworked to improve yield, or discarded, thereby avoiding further processing of known defective substrates. In case only some target portions of the substrate are defective, the further exposure may be performed only on those target portions that are good.

FIG. 1 also shows metrology equipment 240, which metrology equipment 240 is provided for making parameter measurements of a product at a desired platform during a manufacturing process. A common example of a metrology station in a modern lithography production facility is a scatterometer, such as a dark field scatterometer, an angle-resolved scatterometer, or a spectral scatterometer, and the scatterometer can be applied to measure properties of the developed substrate at 220 prior to etching in the apparatus 222. With the use of the metrology apparatus 240, it may be determined that important performance parameters, such as overlay or Critical Dimension (CD), for example, do not meet specified accuracy requirements in the developed resist. Prior to the etching step, there is an opportunity to strip the developed resist through the lithography cluster and rework the substrate 220. With small adjustments over time by the supervisory control system SCS and/or the control unit LACU 206, the metrology results 242 from the tool 240 can be used to maintain accurate performance of the patterning operation in the lithography cluster, thereby minimizing the risk of producing out-of-specification products and requiring rework.

Additionally, metrology device 240 and/or other metrology devices (not shown) may be employed to measure properties of the processed substrates 232, 234 and the incoming substrate 230. Metrology equipment may be used on the processed substrate to determine important parameters such as overlay or CD.

A metrology apparatus suitable for use in embodiments of the present invention is shown in figure 2 (a). This is merely an example of a metrology device and any suitable metrology device may be used (e.g., for performing dark field measurements). The target T and the diffracted rays of the measuring radiation with which the target is irradiated are illustrated in more detail in fig. 2 (b). The illustrated metrology apparatus is of the type known as dark field metrology apparatus. The metrology apparatus may be a separate device or incorporated in the lithographic apparatus LA, for example at a metrology station, or in the lithographic cell LC. The optical axis with several branches throughout the device is indicated by the dashed line O. In this apparatus, light emitted by a source 11 (e.g., a xenon lamp) is directed onto a substrate W via a beam splitter 15 by an optical system comprising lenses 12, 14 and an objective lens 16. The lenses are arranged in a double sequence of a 4F arrangement. Different lens arrangements may be used as long as they still provide a substrate image onto the detector and simultaneously allow access, i.e. access, to the intermediate pupil plane for spatial-frequency filtering. Thus, the angular range over which the radiation is incident on the substrate can be selected by defining a spatial intensity distribution in a plane (referred to herein as the (conjugate) pupil plane) that exhibits the spatial spectrum of the substrate plane. In particular, this selection can be made by inserting an aperture plate 13 of suitable form between the lenses 12 and 14 in the plane of the back-projected image as the pupil plane of the objective lens. In the illustrated example, the aperture plate 13 has different forms (labeled 13N and 13S) allowing different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In the first illumination mode, the aperture plate 13N provides off-axis from a direction designated "north" for the sake of description only. In the second illumination mode, the aperture plate 13S is used to provide similar illumination, but from the opposite direction, labeled "south". Other illumination modes are possible by using different apertures. The illumination mode described in some of the following embodiments is a quaternary illumination mode 13Q, also shown, which separates the higher diffraction orders (e.g., +1 and-1) into diagonally opposite quadrants of the image, with the zeroth order pointing to the other two quadrants. The rest of the pupil plane is ideally dark, since any unwanted light outside the desired illumination mode will interfere with the desired measurement signal.

As shown in fig. 2(b), the target T is placed such that the substrate W is perpendicular to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown in the figure). The ray I of the measuring radiation impinging on the target T at an angle to the axis O gives rise to one zeroth order ray (solid line 0) and two first order rays (dotted lane +1 and dotted lane-1). It should be kept in mind that in the case of an overfilled small target, these rays are the only one of many parallel rays that cover the area of the substrate that includes the metrology target T and other features. Since the holes in the plate 13 have a limited width (i.e. which is necessary to admit a useful amount of light), the incident radiation I will actually occupy a range of angles and the diffracted radiation 0 and +1/-1 will be slightly spread out. According to the point spread function of a small target, each order +1 and-1 will spread further over a range of angles, rather than a single ideal ray as shown. It should be noted that the grating pitch and illumination angle of the target may be designed or adjusted such that the first order rays entering the objective lens are closely aligned with the central optical axis. The rays illustrated in fig. 2(a) and 2(b) are shown slightly off-axis, only to enable them to be more easily distinguished in the figures.

At least the 0 and +1 orders diffracted by the target T on the substrate W are collected by the objective lens 16 and directed back through the beam splitter 15. Returning to fig. 2(a), both the first and second illumination modes are illustrated by designating diametrically opposite apertures labeled north (N) and south (S). When the incident ray I of the measurement radiation comes from the north side of the optical axis, i.e. when the first illumination mode is applied using the aperture plate 13N, the +1 diffracted ray labeled +1(N) enters the objective lens 16. In contrast, when the second irradiation mode is applied using the aperture plate 13S, the-1 diffracted ray (denoted as-1 (S)) is the diffracted ray entering the lens 16.

The second beam splitter 17 divides the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zero-order diffracted beam and the first-order diffracted beam to form a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g., a CCD or CMOS sensor). Each diffracted order hits a different point on the sensor so that the image processing can compare and contrast multiple orders. The pupil plane images captured by the sensor 19 may be used for many measurement purposes, such as reconstruction used in the methods described herein. The pupil plane image may also be used to focus the metrology device and/or to normalize intensity measurements of the first order beam.

In the second measurement branch, the optical system 20, 22 forms an image of the target T on a sensor 23 (for example a CCD or CMOS sensor). In the second measurement branch, the aperture stop 21 is arranged in a plane conjugate to the pupil plane. The aperture stop 21 is used to block the zero order diffracted beam so that the image of the target formed on the sensor 23 is formed by only the-1 or +1 order beam. Alternatively, wedges may be provided to create the split image from the-1 and +1 first order beams simultaneously. The images captured by the sensors 19 and 23 are output to a processor and controller PU, the function of which will depend on the particular type of measurement being performed. It should be noted that the term "image" is used herein in a broad sense. As long as there is one of the-1 and +1 orders, no image of such raster lines will be formed.

The particular form of aperture plate 13 and field stop 21 shown in fig. 2 is merely exemplary. In another embodiment of the invention, on-axis illumination of the target is used, and an aperture stop with an off-axis aperture is used to pass substantially only one first order diffracted light to the sensor. In yet other embodiments, second, third, and higher order beams (not shown in FIG. 2) may also be used in the measurement instead of or in addition to the first order beam.

The target T may include a number of gratings, which may have overlay offsets biased in different ways in order to facilitate measurement of the overlay between the multiple layers that are used to form different portions of the composite grating. The gratings may also differ in their orientation in order to diffract incident radiation in the X-direction and the Y-direction. Discrete images of these gratings may be identified in the image captured by the sensor 23. Once the separate images of the gratings have been identified, the intensity of those separate images may be measured, for example, by averaging or summing selected pixel intensity values within the identified regions. The intensity and/or other properties of the images may be compared to each other. These results may be combined to measure different parameters of the lithographic process.

Generally, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, which requires a high accuracy of dimensioning and placement of structures on the substrate W. To ensure such high accuracy, the three systems may be combined into a so-called "global" control environment, as depicted in fig. 3. One of these systems is a lithographic apparatus LA, which is (actually) connected to a metrology tool MET (second system) and to a computer system CL (third system). The key to this "global" environment is to optimize the cooperation between these three systems to enhance the overall process window and to provide a tight control loop to ensure that the patterning performed by the lithographic apparatus LA remains within a process window. The process window defines a range of process parameters (e.g., dose, focus, overlap) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device) -typically allowing process parameters in a lithographic process or a patterning process to vary within these parameter ranges.

The computer system CL may use (parts of) the design layout to be patterned to predict what resolution enhancement techniques are to be used, and to perform computational lithography simulations and calculations to determine what mask layout and lithographic apparatus settings achieve the maximum overall process window for the patterning process (as depicted in fig. 3 by the double arrow in the first scale SC 1). Typically, resolution enhancement techniques are arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where the lithographic apparatus LA is currently operating within the process window (e.g., using input from the metrology tool MET) to predict whether a defect may be present due to, for example, sub-optimal processing (depicted in fig. 3 by the arrow pointing to "0" in the second scale SC 2).

The metrology tool MET may provide input to the computer system CL to enable accurate simulation and prediction, and may provide feedback to the lithographic apparatus LA to identify possible drifts, for example in a calibrated state of the lithographic apparatus LA (depicted in fig. 3 by the plurality of arrows in the third scale SC 3).

Focus monitoring is an important parameter for proper lithographic apparatus performance. In non-EUV systems, one method for monitoring focus is diffraction-based focusing (DBF). This involves exposing the structure with an intentional focus-dependent asymmetry. By measuring this asymmetry on the exposed structure, the focus at the exposure can be inferred. However, the DBF target (on the reticle) includes sub-resolution structures that may cause defects on the substrate and may not comply with certain design rules. Furthermore, limitations such as thin resist thickness mean that DBFs are not always suitable (or at least more difficult to implement) for EUV systems. Astigmatism-based focusing (ABF) is an alternative approach to DBF, which provides a solution for EUV focus monitoring. However, this method requires the introduction of astigmatism in the imaging lens, which means that it cannot be used for on-product metrology. To address these issues, an optical focus metrology technique will be described that can measure focus parameters (and optionally dose parameters) on a simple line-space target, and that is suitable for on-product applications. A focus parameter and a dose parameter of a structure may describe a focus setting and a dose setting, respectively, of the lithographic exposure apparatus (scanner) when exposing the structure.

The proposed method utilizes cross-polarization modes in the measurement (e.g. scattering) device. By using different cross-polarization modes, different measurement relationships as a function of focal length can be obtained, from which suitable relationships can be identified and used for focus monitoring. A suitable relationship may be a monotonic relationship over a sufficient focus range for focus monitoring of the lithographic process. The content, including the sufficient focus range, will vary between scanner types. For non-EUV scanners, the sufficient focus range around the optimum focus may be 150nm, while for EUV scanners, the sufficient focus range around the optimum focus may be 90 nm. More generally, within the scope of the present disclosure, a sufficient focus range around the optimal focal length may include, for example, any of 250nm, 200nm, 150nm, 120nm, 100nm, 90nm, 70nm, or 50 nm; or any focal range in the range between 200nm and 50 nm.

As such, the metrology device may be operable with an illumination polarization state selected between a first (e.g., horizontal) polarization state and a second (e.g., vertical) polarization state; and the sensor for sensing scattered illumination from the structure is operable in a sensing illumination state selectable between the first and second polarisation states.

In particular, it is proposed to perform a set of polarimetric measurements. Using a standard tagging convention (e.g., as used in radar imaging), a set of polarimetric measurements may yield one, some, or all of HH, VV, HV, and VH polarization state measurement data, where H and V refer to horizontal and vertical polarization states, respectively, and:

HH is horizontal emission (e.g., illumination state) and horizontal reception (e.g., sensing state), VV is vertical emission and vertical reception,

HV is horizontal transmission and vertical reception, and

VH is vertical transmission and horizontal reception.

Typically, each measurement of the set of polarization metrology measurements is obtained in a separate acquisition, for example, by appropriately changing the polarizers within the metrology tool between acquisitions. However, depending on the optical system used, in principle some polarization states can be acquired simultaneously. For example, measurements may be performed simultaneously in HH and HV measurement states and similar VH and VV measurement states, where polarization may be split at the output.

The inventors have observed that different polarization measurement states may each show a completely different focus response, providing a greater chance of finding a suitable monotonic relationship between focal length and measurement data (such as measured intensity data) (e.g., one or more intensity values; e.g., intensity in a pupil plane, and/or a dark field intensity measurement of a diffraction order, for example). In particular, the measurement data may comprise derived or processed measurement data derived from the measured intensities (intensity signal data), more particularly from the measured angle-resolved intensities within the pupil. Such processed signals may include scores or scores for principal components obtained from Principal Component Analysis (PCA) processing of the measurement data (intensity signal data). However, other methods for component analysis, processing, and/or machine learning (e.g., artificial intelligence) algorithms may be used.

Such a method may include an initial calibration or learning phase to create a suitable focus model that may infer focus from measurement data associated with at least one polarimetric measurement. The calibration phase may be based on exposure of an exposure matrix and subsequent polarization measurements or other cross-polarization measurements, such as typically exposure on an FEM wafer (focus exposure matrix or focus energy matrix wafer) to generate calibration measurement data, more specifically, one or more of HV polarization state calibration measurement data, VH polarization state calibration measurement data, HH polarization state calibration measurement data, and VV polarization state calibration measurement data.

The concept of FEM wafers is known. Traditionally, the optimal settings are determined by "send ahead wafer", i.e., the substrate exposed, developed and measured before the production run time. A FEM wafer includes a wafer that has been coated with photoresist on which a pattern is exposed in various combinations of focus and exposure offsets. The FEM wafer is measured by a metrology tool to determine sidewall angle (SWA) and Critical Dimension (CD) using, for example, a reconstruction method. A focus model or focus dose model may then be constructed from these measurements and known focus values (e.g., as set). The focus dose model describes and interpolates the relationship between focus and dose and CD and SWA. With this interpolation model, any single CD and/or SWA measurement can be converted to focus and dose. Such a method is described, for example, in U.S. patent application US2011-0249244, which is incorporated by reference herein in its entirety.

In this proposal, the FEM wafer follows a similar rationale, but includes multiple (e.g., symmetric) line-space gratings in the field exposed using different (known) focus and dose settings. In one embodiment, each field will comprise a plurality of different line-space gratings, which differ in their pitch and/or CD. It is also proposed that the focus model (or focus dose model) is based on intensity measurements rather than measurements that require a complete reconstruction. Although the following main embodiments will describe a focus model, one skilled in the art will recognize that a focus dose model may be created to enable focus and dose monitoring. In such a model, the FEM will include different focal lengths and doses, and a focal length dose model created from the FEM measurements in combination with known focal length and dose values.

Fig. 4 illustrates the advantage of using cross polarization states. Fig. 4(a) and 4(b) show graphs of (largely or mainly unprocessed) measurement data, more particularly the average intensity I (e.g. of the pupil or of the angle-resolved intensity distribution obtained in the measurement) versus the focal length f. In this context, largely unprocessed means not undergoing the data processing described below to produce a suitable monotonic focus response; this data may have undergone conventional processing such as averaging and/or normalization. Fig. 4(a) shows the focus response for HH and VV co-polarized states, and fig. 4(b) shows the focus response for HV and VH cross-polarized states. In each case, the bosch response is observed in a similar manner to existing CD focus metrology techniques, and thus will suffer from the same inherent problems (lack of sensitivity around the best focus bf (i.e., zero defocus), and unsigned or signed information). As such, in this state, these measurements are not particularly useful for creating a focus model. However, with appropriate processing of the intensity data, and in particular the intensity data relating to the HV and VH cross-polarization states (for this example), a suitable monotonic focus response can be obtained.

Fig. 4(c) and 4(d) relate to the same process indicator/measurement data (e.g. intensity data) as shown in fig. 4(a) and 4(b), respectively, but after processing this data. Fig. 4(c) is a graph of processed measurement data (e.g., processed intensity index) Ip versus focal length for HH and VV common polarization states, and fig. 4(d) is a graph of processed measurement data (e.g., processed intensity index) Ip versus focal length for HV and VH cross polarization states. In this particular embodiment, the processing includes performing Principal Component Analysis (PCA), and the graph relates to scores or scores for particular principal components. Here, the graph relates to the second principal component, although any principal component may be used that shows a suitable (e.g. optimal) focus response, or more than one principal component may form the basis of the focus model. As can be seen, while the two HH and VV co-polarized states in fig. 4(c) still show a large degree of bosch response, the HV and VH cross-polarized states in fig. 4(d) each show a monotonic response focused within a suitable (i.e., sufficiently large) focus range around the optimum focal length bf. Thus, such processed data relating to HV and VH cross-polarization states are particularly suitable for creating a focus model.

It will be appreciated that an optimum response will not necessarily be observed for HV and VH cross polarization states. As such, one or both of the HH and VV co-polarized state (processed) signals may also be used to create the focus model (either as an alternative to, or in combination with (one or both of) the HV and VH cross-polarized states), depending on the stack, target and/or acquisition settings. It should be noted that in this particular example, the two graphs HV and VH in fig. 4(d) are almost identical, so that they appear to be one, although this need not necessarily be the case.

It should be understood that PCA is only one example of signal processing that may be used in the methods disclosed herein. Other suitable methods may include, for example, Independent Component Analysis (ICA) or Probabilistic Latent Semantic Analysis (PLSA). Advanced mathematical algorithms, including machine learning algorithms, may be used alternatively or additionally. Many different Artificial Intelligence (AI) techniques, collectively referred to as machine learning, can be utilized. These techniques may be linear, such as Partial Least Squares Regression (PLSR), or non-linear, such as Support Vector Machines (SVMs) using non-linear kernels.

Fig. 5 is a flow chart describing the basic steps of the proposed focus measurement method. The calibration phase 500 includes a FEM exposure step 510. At this step 510, FEM (or alternatively, such as the production substrate exposed at step 550) is exposed using a reticle having a plurality of line-space targets with various pitches and CDs. Various line-space targets may be set to determine one or more preferred targets for production monitoring. Such preferred targets may include targets with good focus sensitivity and low dose crosstalk. Furthermore, two or more CD/pitch combinations may also be used to create a focus model in order to suppress process effects. The model may be trained for (e.g., expected) process variations (e.g., multiple targets are affected by different process variations during their formation) to provide process robustness, i.e., robustness (to a target or a combination of targets) in the model. In this way, it is proposed to improve and simplify the focus model by providing such a diversity of targets, thereby providing a focus model that is more robust to dose and/or process variations.

At step 520, FEM is measured using a cross-polarization or polarization metrology function to obtain measurements for at least one cross-polarization state (e.g., HV and/or VH states). Preferably, this step will produce an equivalent measurement in each of the HV and VH states, and more preferably still in each of the HV, VH, HH and VV states (fully polarized measurement).

At step 530, a focus model is created based on the measurements performed at step 520 and the known focus values (e.g., actual focus settings from the lithographic apparatus during FEM exposure). The focus model may be created using measurements related to two cross-polarization states, although it is also possible and within the scope of the invention to create a model from only one of the cross-polarization states (e.g., the state showing the best monotonic focus response). Measurements related to the common polarization state may also be used to create the focus model (either in combination with or instead of the observed related focus response). As explained above, this step may include an initial processing step to obtain a suitable monotonic focus response for at least one polarization state. Any of the processing methods described above may be used, such as PCA, ICA, PLSA, PSLR, SVM, or any other suitable processing method.

In the production phase 540, the production substrate (or other focus monitoring substrate) is exposed 550 with one or more targets having the same features as used to create the focus model. At step 560, a focus monitoring measurement is performed on the target using the same polarization mode (e.g., polarimetric measurement) as was used to create the focus model. At step 570, the focus model created at step 530 is used to infer a focus value (of the lithographic apparatus when forming a target) from measurement data (e.g. intensity data). For the specific example illustrated by fig. 4, the inference may be based on, inter alia, measurement data corresponding to HV and VH cross polarization states; however, this will depend on which polarization state or states show the best focus response for a particular situation, and therefore a focus model has been created for it.

It should be noted that metrology can include measurements of structures formed in the resist (ADI after development inspection) or after etching (AEI after etch inspection). In the case of an AEI, steps 510 through 530 would need to be performed similarly after etching to obtain a post-etch focus model.

Additional embodiments of the invention are disclosed in the following list of numbered aspects: 1. a method of measuring focus parameters associated with forming a structure using a lithographic process, wherein the method comprises:

obtaining measurement data relating to cross-polarization measurements of the structure; and

determining a value of the focus parameter based on the measurement data.

2. The method defined in aspect 1, wherein the measurement data comprises one or more of HV polarization state measurement data, VH polarization state measurement data, HH polarization state measurement data, and VV polarization state measurement data.

3. The method as defined in aspect 2, wherein the measurement data comprises at least the HV polarization state measurement data and/or the VH polarization state measurement data.

4. A method as defined in aspect 3, wherein the measurement data comprises the HH polarization state measurement data and/or the VV polarization state measurement data.

5. The method defined in any of the preceding aspects, wherein the structure comprises a line-space grating.

6. The method as defined in any of the preceding aspects, wherein the line-space grating is designed to be substantially symmetric.

7. A method as defined in any preceding aspect, comprising performing the cross-polarization measurement on the structure on a production substrate.

8. The method defined in any of the preceding aspects, wherein the determining step is performed using a focus model.

9. The method defined in aspect 8 wherein the calibration measurement data relating to cross-polarization calibration measurements of the exposure matrix is used to create the focus model in the calibration phase, the exposure matrix including at least the calibration structure exposed in multiple exposures having a plurality of different focus offsets.

10. A method as defined in aspect 9, wherein the exposure matrix includes multiple exposures with multiple different dose offsets, and the step of creating a focus model includes creating a focus dose model that is further operable to enable determination of dose parameters based on the measurement data.

11. The method defined in aspect 9 or 10 wherein each exposure of the plurality of exposures comprises a plurality of calibration structures, each calibration structure comprising a line-space grating, wherein the pitch and/or critical dimension of the calibration structures is varied.

12. The method defined in any one of aspects 9 to 11 wherein the calibration measurement data comprises one or more of the following calibration measurement data: HV polarization state calibration measurement data related to HV polarization state calibration measurements, VH polarization state calibration measurement data related to VH polarization state calibration measurements, HH polarization state calibration measurement data related to HH polarization state calibration measurements, and VV polarization state calibration measurement data related to VV polarization state calibration measurements.

13. The method defined in aspect 12 wherein the cross-polarization calibration measurement data comprises at least two of the following cross-polarization calibration measurement data: HV polarization state calibration measurement data, VH polarization state calibration measurement data, HH polarization state calibration measurement data, and VV polarization state calibration measurement data.

14. The method defined in aspect 13, wherein the calibration measurement data comprises at least HV polarization state calibration measurement data and/or VH polarization state calibration measurement data.

15. The method defined in aspect 14, wherein the calibration measurement data further comprises HH polarization state calibration measurement data and/or VV polarization state calibration measurement data.

16. The method as defined in any of aspects 13 to 15, wherein calibration measurement data used to calibrate the focus model is weighted to favor or only include the calibration measurement data in relation to any of the polarization states that have a monotonic relationship with focus parameters at least over a focus range around a best focus distance sufficient to monitor focus during a lithographic process, whether after a processing step or otherwise.

17. The method defined in any one of aspects 8 to 16 comprises the step of processing the calibration measurement data to obtain processed calibration measurement data having a monotonic relationship to the focus parameter at least over a focus range around the optimum focus sufficient to monitor focus during a lithographic process.

18. A method as defined in aspect 17, wherein the processed calibration measurement data comprises: one or more principal or independent components of the calibration measurement data after principal or independent component analysis, and/or a score of the one or more principal or independent components.

19. The method defined in aspect 17 or 18 wherein the processing comprises applying one or more machine learning algorithms, such as partial least squares regression or a non-linear support vector machine algorithm.

20. The method defined in any one of aspects 16 to 19, wherein the focus range around the optimal focal length sufficient to monitor focus in the lithographic process comprises a range of 200nm to 50nm around the optimal focal length.

21. A computer program comprising program instructions operable, when run on a suitable device, to perform the method of any of aspects 1 to 20.

22. A non-transitory computer program carrier comprising a computer program according to aspect 21.

23. A processing system comprising a processor and a computer program according to aspect 22.

24. A metrology system, comprising:

a substrate holder for a substrate;

an illumination source for illuminating structures on the substrate with radiation having an illumination polarization state selectable between a first polarization state and a second polarization state;

a sensor for sensing scattered illumination from the structure with a sensing illumination state selectable between the first and second polarization states; and

the processing system of aspect 23.

25. The metrology system of aspect 24, wherein the first polarization state is a horizontal polarization state and the second polarization state is a vertical polarization state.

Although the above description describes corrections for a lithographic apparatus/scanner, the determined corrections may be used for any process and by any Integrated Circuit (IC) manufacturing apparatus in an IC manufacturing process, such as an etching apparatus that has an effect on the position and/or size of structures formed within a layer.

The terms "radiation" and "beam" used in connection with lithographic apparatus encompass all types of electromagnetic radiation, including Ultraviolet (UV) radiation (e.g. having a wavelength of or about 365nm, 355nm, 248nm, 193nm, 157nm or 126nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5nm-20 nm), as well as particle beams, such as ion beams or electron beams.

The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments without undue experimentation, without departing from the general concept of the present invention. Therefore, these adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented in this disclosure. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.

Claims (15)

1. A method of measuring focus parameters associated with forming a structure using a lithographic process, wherein the method comprises:

obtaining measurement data relating to cross-polarization measurements of the structure; and

determining a value of the focus parameter based on the measurement data.

2. The method of claim 1, wherein the measurement data comprises one or more of HV polarization state measurement data, VH polarization state measurement data, HH polarization state measurement data, and VV polarization state measurement data.

3. A method according to any preceding claim, wherein the structure comprises a line-space grating.

4. The method of any preceding claim, wherein the line-space grating is designed to be substantially symmetrical.

5. The method of any preceding claim, comprising: performing the cross-polarization measurement on the structure on a production substrate.

6. The method of any preceding claim, wherein the determining step is performed using a focus model.

7. The method of claim 6, wherein calibration measurement data relating to cross-polarization calibration measurements of an exposure matrix comprising at least calibration structures exposed in multiple exposures with multiple different focus offsets is used to create a focus model in a calibration phase.

8. The method of claim 7, wherein the exposure matrix comprises multiple exposures with multiple different dose offsets, and the step of creating a focus model comprises creating a focus dose model further operable to enable determination of dose parameters based on the measurement data.

9. The method of claim 7 or 8, wherein each exposure of the plurality of exposures comprises a plurality of calibration structures, each calibration structure comprising a line-space grating, wherein the pitch and/or critical dimension of the calibration structures is varied.

10. The method of any of claims 7 to 9, wherein the calibration measurement data comprises one or more of the following calibration measurement data: HV polarization state calibration measurement data related to HV polarization state calibration measurements, VH polarization state calibration measurement data related to VH polarization state calibration measurements, HH polarization state calibration measurement data related to HH polarization state calibration measurements, and VV polarization state calibration measurement data related to VV polarization state calibration measurements.

11. A method according to any of claims 6 to 10, comprising the step of processing the calibration measurement data to obtain processed calibration measurement data which is monotonically related to a focus parameter at least over a focus range around a best focus sufficient to monitor focus during lithography; and is

Optionally, the processed calibration measurement data includes: one or more principal or independent components of the calibration measurement data after principal or independent component analysis, and/or a score of the one or more principal or independent components.

12. A computer program comprising program instructions operable, when run on a suitable device, to perform the method of any of claims 1 to 11.

13. A non-transitory computer program carrier comprising a computer program according to claim 12.

14. A processing system comprising a processor and a computer program according to claim 12.

15. A metrology system, comprising:

a substrate holder for a substrate;

an illumination source for illuminating structures on the substrate with radiation having an illumination polarization state selectable between a first polarization state and a second polarization state;

a sensor for sensing scattered illumination from the structure with a sensing illumination state selectable between the first and second polarization states; and

the processing system of claim 14.

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